1. Field of the Invention
The invention relates to brake systems of heavy duty vehicles, and in particular to cam assemblies of the brake system. More particularly, the invention is directed to a cam shaft enclosure/support assembly which enables efficient installation of the cam assembly on various types of axle/suspension systems, and extends the life of the cam assembly.
2. Background Art
In conventional heavy duty vehicle brake systems, an S-cam is utilized to lift brake shoes against a brake drum to decelerate a vehicle. The cam shaft to which the S-cam is integrally connected typically is supported at each of its ends by a bushing or bearing. These bushings usually are greased to reduce friction between the bushings and the cam shaft.
If the bushings or the cam shaft become worn and/or rotational friction of the cam shaft against the bushings increases, the overall efficiency of the brake system decreases. Of course, when bushing or cam shaft wear exceeds predetermined limits, bushing and/or cam shaft replacement is required. Recommended practice in the industry is that when the cam shaft and/or bushings on one end of an axle require replacement, then the cam shaft and/or bushings on the other end of the same axle should also be replaced, and all other axles of the vehicle should be inspected for the same worn condition.
There are two primary contributors to cam shaft and bushing wear, namely, load-induced wear and contamination-induced wear. Wear due to loading is encountered in two different conditions that can be defined as quasi-static and dynamic. The quasi-static case occurs when the vehicle brakes are applied and braking forces are reacted in the cam shaft bushings. This scenario is considered quasi-static due to the relatively low rotational speed of the cam shaft and the steady state condition when the brakes are held at constant pressure. In such a case, the loads on the outboard bushing are greater than on the inboard bushing. The dynamic load case typically occurs when the brakes are in the released condition and the cam shaft experiences vibrations due to road inputs. These vibrations result in impact loading of the cam shaft against the bushings.
Cam shaft and bushing wear due to contamination is caused primarily by environmental factors. Seals generally are disposed on each end of each bushing to capture the lubricant inside the bushings as well as to prevent ingress of contaminants from the outside environment. The lubricant not only acts to reduce friction between the cam shaft and the bushings, but also suspends any contaminants that may migrate past the seals and into the bushing. The lubricant also acts as a barrier to moisture that could cause corrosion of the cam shaft.
Thus, various types of cam shaft enclosure and/or support assemblies have been utilized in the brake system art to protect and support the cam shaft and ensure coaxiality of the bushings to prevent excessive rotational friction or binding of the cam shaft against one or more of the bushings when the brakes are actuated. Maintaining coaxiality of the bushings also improves the load support of the bushings. More particularly, loads are more evenly distributed across the surfaces of both bushings and there is a reduced chance of the cam shaft contacting a small area or edge of one of the bushings. Such support increases the load-bearing area available for the cam shaft, thus reducing bushing wear due to quasi-static and dynamic load conditions. Such a cam support/enclosure assembly also protects the bushings from environmental contamination. The cam tube eliminates two seals over bushings used without a cam tube, and reduces by two the number of locations for ingress of contaminants into the bushings. The cam tube also provides a larger grease reservoir to improve lubricity and to suspend any contaminants that may migrate past the seals.
However, prior art cam shaft support/enclosure assemblies typically have required a weld at the attachment point of the inboard end of the cam tube to the vehicle to react rotation of the tube induced by cam shaft rotation. Unfortunately, such a weld can be subject to fatigue and failure. Moreover, due to the requirement of such welding and/or shimming during installation of the cam tube support/enclosure assembly during production of an axle/suspension system, the cam assembly typically must be custom fit to a single type of axle/suspension system. More specifically, different axle/suspension systems have different distances between the two major support points for the cam assembly, namely, the brake system spider and the beam of the axle/suspension system. Thus, one size of cam assembly with fixed weld points will fail to fit many axle/suspension systems. In addition, custom-fitting also is required on same-type axle/suspension systems due to small tolerances in the distance between the support points for the cam tube caused by natural variations in manufacturing processes.
The present invention solves the above-described problems of fatigue failure and custom-fitting by utilizing an inboard cam tube support plate having a predetermined keyhole configuration that matches the configuration of the periphery of the inboard end of the cam tube. The support plate reacts cam tube rotation without the possibility of fatigue to and failure of a weld. The keyhole slip fit between the cam tube and support plate further enables installation of the cam tube on different types of axle suspension systems, where the distance between the brake spider and inboard suspension beam points of support for the cam tube varies, as well as on same-type suspension assemblies without concern for differing distances between the support points caused by natural variations in manufacturing processes.